Some mythical creatures, such as the dragon and griffin, have their origin in traditional mythology and have been believed to represent (in some form) a real creature.
Other mythical creatures were attempts to describe real creatures, originating in garbled accounts of travelers' tales, leading to an inaccurate description of a real creature.
Similarly (but fundamentally different), some mythical creatures are hybrids, a combination of two or more animals which may have been for descriptive reasons or simple literary license.